Exposure to cigarette smoke is a major cause of life-threatening diseases like bronchitis, emphysema, other diseases of the respiratory tract, coronary heart diseases, lung cancer and other malignancies [1-5]. In fact, cigarette smoke is the overwhelming cause of lung cancer, now the most common cancer globally. Since approaches to cessation of smoking by public health campaigns and anti-smoking laws passed by local Governments have had limited success, the most practicable approach is the prevention of the hazardous effects caused by cigarette smoke. Cigarette smoke in known to contain about 4000 components, out of which about 3000 components are present in the gas phase and about 1000 components in the tar phase [6]. The oxidants in the gas phase, such as O−2, H2O2, NO, peroxy radical are extremely unstable [7]. If the gas phase is passed into phosphate buffer and the resultant solution is added to albumin solution, no protein oxidation occurs (7). Apparently, any damage caused by the gas phase is expected to be restricted to the buccal cavity and upper respiratory tract [8]. On the other hand, the oxidant(s) present in the tar are quite stable and these are apparently responsible for producing oxidative damage in the lung, heart and other organs [7,9]. About 48 per cent of the tar components are water soluble [10] and the aqueous extract of tar is known to produce oxidative damage of biological macromolecules including proteins and DNA [7,11,12]. However, it is perplexing to conceive how many of the components present in the aqueous extract of tar are responsible for producing oxidative damage in the biological system. Uptil now, among the many components of cigarette smoke, three classes of compounds have been suggested to be implicated as causative agents in the development of cancer and degenerative diseases, namely, (i) polycyclic aromatic hydrocarbons (ii) nitrosamines and (iii) free radicals.
Among the polycyclic hydrocarbons, benzo [a] pyrene is by far the best studied. But it is not a carcinogen and requires metabolic activation through cytochrome P450 system to become the ultimate carcinogen, benzo [a] pyrene diol epoxide. Moreover, the concentration of benzo [a] pyrene in cigarette smoke is meagre, about 10 to 40 ng per cigarette [13] and benzo [a] pyrene cannot explain oxidative damage of protein produced by cigarette smoke.
Among the tobacco specific nitrosamines (TSNA), the most studied ones are N1-nitrosonomicotine (NNN) and 4-(methylnitrasamino)-1-(3-pyridyl)-1-butanone (NNK). Again TSNA are not direct carcinogens and also their concentrations in tobacco smoke vary widely. The observed range for NNN is 0.004 μg to 1.35 μg and for NNK, <0.004 μg to 1.75 μg per cigarette. It is concluded that TSNA in cigarette smoke is not a sufficient index for the carcinogenic potential of cigarette smoke [14]. Again TSNA cannot explain oxidative damage of proteins.
Another aspect of the hazardous component of cigarette smoke is free radical. Pryor and his associates made considerable studies on free radical chemistry of cigarette smoke and its toxicological implications. These authors suggest that the principal relatively stable free radical in cigarette tar may be a quinone/hydroquinone complex which is an active redox system and that this redox system is capable of reducing molecular oxygen to produce superoxide, leading to hydrogen peroxide and hydroxyl radicals [15], that may eventually lead to oxidative damage of biological macromolecules but we have observed that oxidative damage of proteins produced by the stable tar radicals is not inhibited by SOD or catalyst indicating that the oxidative damage is not mediated by super oxide radical or hydrogen peroxide. The applicants have further observed that the tar radicals oxidize proteins in nitrogen atmosphere and in the absence of molecular oxygen, indicating a direct interaction of the tar radicals with biological micromolecules. However, these authors admit that the principal radical they have identified in tar is actually not a monoradical and probably is not a single species (16). They also admit that cigarette tar is an incredibly complex mixture and since the tar radicals have not been isolated and unambiguously identified, any conclusion concerning the chemistry or biochemistry of the tar radicals must be regarded as tentative [15].
It is noteworthy to mention that by the 1960s, the tobacco industry in general had proven in its own laboratory that cigarette tar causes cancer in animals [17]. Throughout 1960s the companies' researchers tried to discover the toxic elements in cigarette smoke with the conviction that if the toxic components could be identified, these agents could be removed or eliminated and a “safe” cigarette could be created, which would deliver nicotine without delivering the toxic substances [17]. But by the late 1970s, the tobacco industry had largely abandoned this particular research, because the objective proved to be unattainable. It was a problem technically difficult to solve and proved untractable [17].
Very recently, we have observed that aqueous extract of whole cigarette smoke/tar contains a major harmful oxidant in relatively high amount, approximately 190±10 μg per cigarette. The applicants have isolated the oxidant, determined the structure and found it to be p-benzosemiquinone. The oxidant almost quantitatively accounts for the oxidative damage of proteins produced by the aqueous extract of whole cigarette smoke/tar. The oxidant is also responsible for DNA oxidation. Nagata et al. (18) have shown that semiquinone radicals bind to DNA and damage it. It is also known that oxidative damage of DNA is implicated with mutation and cancer. The oxidant is relatively stable. Its half-life in the solid state at room temperature is approximately 48 hours. The presence of the stable oxidant in cigarette smoke would explain the deleterious effects of side stream smoke and passive smoking (7). The oxidant is absent in nonsmoking tobacco and is produced during burning of the cigarette (7). Applicants have identified a number of chemical compounds/agents those deactivate the oxidant and may be used as antidotes.